Traditionally, valve actuators have been provided with one or more springs which bias a piston connected to a valve shaft to close the valve in the event of an emergency situation or in the case of hydraulic failure. Typically in these actuators, hydraulic forces are applied to an opposing side of a piston to move the valve into an open position and at the same time compress the return spring or springs. Typical of such actuators is an actuator made by Otis Engineering Corporation, Model WC. The problem with existing actuators is that if there is a wireline going through the open valve, a force of at least 7-8,000 lbs. is necessary to be applied to the valve actuator shaft to ensure that wirelines up to 7/32nds of an inch can be efficiently cut to seal off the well. This additional force is necessary close to the completion of the stroke of the valve actuator shaft since as the valve actuator shaft moves, the wireline is displaced until the valve is almost closed. At that point, the valve gate bears against the wireline in order to cut it off. The problem with prior designs has been that the internal spring is almost fully extended back to its extended position at the time when the maximum force for cutting the wireline is required. To compensate for this, prior designs like the Otis Model WC actuator have put in oversized springs such that at the time the necessary 7-8,000 lbs. is required for wire cutting, the spring still retains that much force at its then-current position within the actuator. This type of design forces the springs to be oversized such that when they are fully compressed (when the valve is open), they store approximately 20,000 lbs. of force. When these springs get to the critical point where the cutting force is necessary, they still retain approximately 8,000 lbs. of stored energy. This generally occurs when the spring reaches approximately 1-1/4 inches short of its fully extended position. Similar designs to the Otis WC actuator have been put out by Vetco-Gray, which has a Graysafe reduced height hydraulic actuator (RHA), which uses a volute spring instead of a coil spring, and by AVA International in its SRM actuator and OOP and OOH models.
Several designs have addressed a need to obtain an incremental force, unleashed toward the end of the actuator stem stroke. One such design is shown in U.S. Pat. No. 4,372,333, where the actuator stem has a tapered surface and there are radially disposed plungers which displace springs when the valve is moved toward an open position. As the valve closes, the plungers traverse the tapered surface and the retained spring force is transferred through the plungers to the actuator stem to boost the force applied to the actuator stem as the valve closes. Similarly, U.S. Pat. No. 4,519,575 uses the same principle but combines in one area the mainsprings used to urge the valve stem up and down and the auxiliary springs which take effect toward the end of the stem stroke. Other devices are known to apply an incremental force to a shaft once the shaft has reached an obstruction. Typical of these devices are U.S. Pat. No. 3,320,861. U.S. Pat. No. 4,523,639 illustrates the use of hydraulic force to move a piston, followed by release of spring force against the collet to lock a ram shaft in the closed position. U.S. Pat. No. 3,272,087 illustrates a device which employs a hydraulic ram with pivoting collets used to stop the pushing force on the ram shaft if the ram encounters a fixed object.